As the light sources for an apparatus that displays color images, e.g., a printer or a projection television, light sources corresponding to three colors that are namely Red (R), Green (G), and Blue (B) are required. In recent years, as these light sources, wavelength converting laser apparatuses (laser oscillators) have been developed in which laser beams in a 900 nanometer band, a 1 micrometer band, and a 1.3 micrometer band are used as fundamental wave laser beams so that, by using a nonlinear material, the fundamental wave laser beams are converted into second harmonic waves each having half the wavelength (twice the frequency) through a Second Harmonic Generation (SHG).
To efficiently extract the laser beams having the desired wavelengths during the SHG, it is necessary to realize a high level of efficiency in the conversion from the fundamental wave laser beams to the second harmonic laser beams. To convert the fundamental wave laser beams into the second harmonic laser beams within a wavelength converting element, phase matching conditions need to be satisfied between the fundamental wave laser beams before the conversion and the second harmonic waves after the conversion. The phase matching conditions are conditions under which phase shifting between the fundamental wave laser beams and the second harmonic laser beams is corrected by the wavelength converting element. As an example of elements that are capable of converting wavelengths while satisfying such phase matching conditions, Quasi Phase Matching (QPM) elements in which a periodic structure is used are known. In a QPM wavelength converting element, an optical waveguide is formed in, for example, Periodically Poled Lithium Niobate (PPLN), which is a nonlinear optical crystal, so that polarization is periodically inverted along the waveguide direction.
Such a QPM wavelength converting element, however, has a small tolerance (a phase matching bandwidth) for the phase matching conditions with respect to the wavelengths of the fundamental waves and the temperature of the wavelength converting element. To cope with this situation, a QPM wavelength converting element having a structure in which the pitch of the polarization inversion periods is gradually changed (a structure in which the periodic structure of the polarization inversions is changed in a chirp-like manner) has been proposed, as a means for broadening the phase matching bandwidth.
An optical wavelength converting element described in Patent Document 1 has a periodical polarization inversion structure that is formed in a nonlinear optical crystal, the polarization inversion structure including a portion having a single period (a single period portion) and a chirp period portion in which the period gradually changes.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-321610